The present invention relates to a single crystal wafer of lithium tantalate or, more particularly, to a single crystal wafer of lithium tantalate suitable as a base material for the preparation of SAW devices having well controlled performance and quality.
As is known, single crystals of lithium tantalate, lithium niobate, quartz and the like are useful as a ferroelectric material and piezoelectric material and, for example, so-called SAW devices are practically manufactured from these single crystal materials. Among these single crystal materials, in particular, lithium tantalate has a relatively high electromechanical coupling coefficient and small temperature dependency of various characteristic properties so that single crystals of lithium tantalate are useful as a material of VCR resonators and PIF filters of television sets as well as some devices for high-frequency communication instruments.
Single crystals of lithium tantalate are usually prepared by the Czochralski method in which the single crystal is grown on a seed crystal pulled up from the melt of an oxide mixture having a composition corresponding to LiTaO.sub.3. The single crystal boule as grown is first subjected to a poling treatment to align the ferroelectric domains into a single-domain body which is then mechanically worked into a cylindrical form and sliced into wafers having a definite crystallographic orientation. The wafer is lapped and polished on the surface before it is used as a base plate of various kinds of devices.
For example, SAW devices are manufactured by cutting such a wafer polished and provided with an electrode of, mostly, aluminum on the surface into small square or rectangular chips. The number of the chips obtained by cutting a single wafer may be only one or may be plural depending on the diameter of the wafer and the dimensions of the desired devices. For example, it is not rare that 200 or more of devices can be manufactured by cutting a single wafer having a diameter of 3 inches. It is of course that the productivity of this process can be increased as the number of the devices taken from a single wafer is increased so that single crystal wafers having larger and larger diameters are desired as a trend toward 3 inches, 4 inches or larger diameters from the 2-inch wafers quite acceptable several years ago.
Such a recent trend of manufacturing a larger and larger number of devices from a single wafer of lithium tantalate single crystals is accompanied by a problem that, along with the increased requirement for uniform and well-controlled performance and quality of the SAW devices obtained from a single wafer, some of the characteristic parameters of the single crystal wafer vary considerably from portion to portion of a wafer and the variation of the values is more remarkable when the diameter of the wafer is larger. The most serious problem in this regard is the variation in the values of the optical properties or double refraction which is one of the most important characteristic parameters in the single crystal wafers of lithium tantalate for manufacturing SAW devices.